An apochromatic lens is one which reduces spherical and chromatic aberrations. (The first part is where stuff looks a weird shape in the photo despite being fine in reality and the other bit is that weird purple haze you sometimes see round the edges of photographic subjects.)
Although that definition gives me a reasonable excuse to post a photo of The Sphere, I have resisted. Instead I've used my own apochromatic lens to photograph a grapefruit. See, no aberrations!
Yes, I am married to a photographer and yes, the poor chap has been interogated mercilessly as I've worked on my latest novel, which features a photographer. Thanks for asking.